Splanchnic Sympathetic Nerve Activity Research Articles

Activation of the hypothalamic paraventricular nucleus by forebrain hypertonicity selectively increases tonic vasomotor sympathetic nerve activity.

We recently reported that mean arterial pressure (MAP) is maintained in water-deprived rats by an irregular tonic component of vasomotor sympathetic nerve activity (SNA) that is driven by neuronal activity in the hypothalamic paraventricular nucleus (PVN). To establish whether generation of tonic SNA requires time-dependent (i.e., hours or days of dehydration) neuroadaptive responses or can be abruptly generated by even acute circuit activation, forebrain sympathoexcitatory osmosensory inputs to PVN were stimulated by infusion (0.1 ml/min, 10 min) of hypertonic saline (HTS; 1.5 M NaCl) through an internal carotid artery (ICA). Whereas isotonic saline (ITS; 0.15 M NaCl) had no effect (n = 5), HTS increased (P < 0.001; n = 6) splanchnic SNA (sSNA), phrenic nerve activity (PNA), and MAP. Bilateral PVN injections of muscimol (n = 6) prevented HTS-evoked increases of integrated sSNA and PNA (P < 0.001) and attenuated the accompanying pressor response (P < 0.01). Blockade of PVN NMDA receptors with d-(2R)-amino-5-phosphonovaleric acid (AP5; n = 6) had similar effects. Analysis of respiratory rhythmic bursting of sSNA revealed that ICA HTS increased mean voltage (P < 0.001) without affecting the amplitude of inspiratory or expiratory bursts. Analysis of cardiac rhythmic sSNA likewise revealed that ICA HTS increased mean voltage. Cardiac rhythmic sSNA oscillation amplitude was also increased, which is consistent with activation of arterial baroreceptor during the accompanying pressor response. Increased mean sSNA voltage by HTS was blocked by prior PVN inhibition (muscimol) and blockade of PVN NMDA receptors (AP5). We conclude that even acute glutamatergic activation of PVN (i.e., by hypertonicity) is sufficient to selectively increase a tonic component of vasomotor SNA.

Abstract 17534: Afferent Vagal Nerve Stimulation Dose Dependently Inhibits Sympathetic Nerve Activity Through the Resetting of Baroreflex Central Arc

Background: Arterial baroreflex is a powerful regulator of the sympathetic nerve activity (SNA) and arterial pressure (AP). Although vagal nerve stimulation (VNS) has been developed as a neuro-modulatory therapy for heart failure, the impact of VNS on baroreflex and SNA remains unknown. Since the afferent fibers of vagal nerves and of baroreceptors converge in the nucleus tractus solitarii, we investigated how the selective afferent VNS (AVNS) impacts on baroreflex in regulating SNA and AP. Methods: We used 6 anesthetized Sprague-Dawley rats (530±11g). We vascularly isolated bilateral carotid sinuses and servo-controlled intra-sinus pressure (CSP). We measured AP, CSP and splanchnic SNA simultaneously. We attached an electrode to the vagal nerve. We established the baseline intensity (voltage) of VNS that reduced heart rate by 10-20%. We then sectioned the vagal nerve at the caudal side of the electrode. We conducted AVNS at 3 doses, baseline, 2xbaseline and 4xbaseline. We changed CSP stepwise from 60 to 160mmHg to analyze static baroreflex function with/without AVNS. To estimate dynamic baroreflex function, we perturbed CSP using binary random sequence (mean AP±20mmHg) and compared the transfer functions (TF) with/without AVNS. Results: AVNS dose-dependently shifted the CSP-SNA relation downward (p&lt;0.05, Fig. 1) without changing the maximum gain, while did not affect the SNA-AP relation (Fig. 2). The TF of CSP-SNA relation approximated a high pass filter characteristics. AVNS dose-dependently decreased the dynamic gain (Control: 0.72±0.20, baseline: 0.46±0.14, 2xbaseline: 0.38±0.12, 4xbaseline: 0.20±0.08 %/mmHg, p&lt;0.05), whereas did not affect the TF of SNA-AP relation. Conclusions: AVNS dose-dependently suppresses SNA by resetting the static CSP-SNA relation, i.e., the baroreflex central arc. However, AVNS reduces the dynamic gain of baroreflex central arc. We conclude that AVNS lowers SNA at the expense of compromising dynamic AP stabilization.

Respiratory modulation of sympathetic activity is attenuated in adult rats conditioned with chronic hypobaric hypoxia

Respiratory modulation of sympathetic nerve activity (SNA) depends on numerous factors including prior experience. In our studies, exposing naïve adult, male Sprague-Dawley rats to acute intermittent hypoxia (AIH) enhanced respiratory-modulation of splanchnic SNA (sSNA); whereas conditioning them to chronic hypobaric hypoxia (CHH) attenuated modulation. Further, AIH can evoke increased SNA in the absence phrenic long-term facilitation. We hypothesized that AIH would restore respiratory modulation of SNA in CHH rats. In anesthetized, CHH-conditioned (0.5 atm, 2 wks) rats (n=16), we recorded phrenic and sSNA before during and after AIH (8% O2 for 45s every 5min for 1h). At baseline, sSNA was not modulated with respiration. The sSNA was not recruited during a single brief exposure of hypoxia nor after 10 repetitive exposures. Further, the sSNA chemoresponse was not restored 1h after completing AIH. Thus, CHH-conditioning blocked the short-term plasticity expressed in sympatho-respiratory efferent activities and this was associated with reduced respiratory modulation of sympathetic activity and with attenuation of the sympatho-respiratory chemoresponse.

Activation of corticotropin-releasing factor receptors in the rostral ventrolateral medulla is required for glucose-induced sympathoexcitation.

Energy expenditure is determined by metabolic rate and diet-induced thermogenesis. Normally, energy expenditure increases due to neural mechanisms that sense plasma levels of ingested nutrients/hormones and reflexively increase sympathetic nerve activity (SNA). Here, we investigated neural mechanisms of glucose-driven sympathetic activation by determining contributions of neuronal activity in the hypothalamic paraventricular nucleus (PVN) and activation of corticotropin-releasing factor (CRF) receptors in the rostral ventrolateral medulla (RVLM). Glucose was infused intravenously (150 mg/kg, 10 min) in male rats to raise plasma glucose concentration to a physiological postprandial level. In conscious rats, glucose infusion activated CRF-containing PVN neurons and TH-containing RVLM neurons, as indexed by c-Fos immunofluorescence. In α-chloralose/urethane-anesthetized rats, glucose infusion increased lumbar and splanchnic SNA, which was nearly prevented by prior RVLM injection of the CRF receptor antagonist astressin (10 pmol/50 nl). This cannot be attributed to a nonspecific effect, as sciatic afferent stimulation increased SNA and ABP equivalently in astressin- and aCSF-injected rats. Glucose-stimulated sympathoexcitation was largely reversed during inhibition of PVN neuronal activity with the GABA-A receptor agonist muscimol (100 pmol/50 nl). The effects of astressin to prevent glucose-stimulated sympathetic activation appear to be specific to interruption of PVN drive to RVLM because RVLM injection of astressin prior to glucose infusion effectively prevented SNA from rising and prevented any fall of SNA in response to acute PVN inhibition with muscimol. These findings suggest that activation of SNA, and thus energy expenditure, by glucose is initiated by activation of CRF receptors in RVLM by descending inputs from PVN.

Afferent vagal nerve stimulation resets baroreflex neural arc and inhibits sympathetic nerve activity.

It has been established that vagal nerve stimulation (VNS) benefits patients and/or animals with heart failure. However, the impact of VNS on sympathetic nerve activity (SNA) remains unknown. In this study, we investigated how vagal afferent stimulation (AVNS) impacts baroreflex control of SNA. In 12 anesthetized Sprague–Dawley rats, we controlled the pressure in isolated bilateral carotid sinuses (CSP), and measured splanchnic SNA and arterial pressure (AP). Under a constant CSP, increasing the voltage of AVNS dose dependently decreased SNA and AP. The averaged maximal inhibition of SNA was ‐28.0 ± 10.3%. To evaluate the dynamic impacts of AVNS on SNA, we performed random AVNS using binary white noise sequences, and identified the transfer function from AVNS to SNA and that from SNA to AP. We also identified transfer functions of the native baroreflex from CSP to SNA (neural arc) and from SNA to AP (peripheral arc). The transfer function from AVNS to SNA strikingly resembled the baroreflex neural arc and the transfer functions of SNA to AP were indistinguishable whether we perturbed ANVS or CSP, indicating that they likely share common central and peripheral neural mechanisms. To examine the impact of AVNS on baroreflex, we changed CSP stepwise and measured SNA and AP responses with or without AVNS. AVNS resets the sigmoidal neural arc downward, but did not affect the linear peripheral arc. In conclusion, AVNS resets the baroreflex neural arc and induces sympathoinhibition in the same manner as the control of SNA and AP by the native baroreflex.

Blood pressure is maintained during dehydration by hypothalamic paraventricular nucleus-driven tonic sympathetic nerve activity.

Resting sympathetic nerve activity (SNA) consists primarily of respiratory and cardiac rhythmic bursts of action potentials. During homeostatic challenges such as dehydration, the hypothalamic paraventricular nucleus (PVN) is activated and drives SNA in support of arterial pressure (AP). Given that PVN neurones project to brainstem cardio-respiratory regions that generate bursting patterns of SNA, we sought to determine the contribution of PVN to support of rhythmic bursting of SNA during dehydration and to elucidate which bursts dominantly contribute to maintenance of AP. Euhydrated (EH) and dehydrated (DH) (48 h water deprived) rats were anaesthetized, bilaterally vagotomized and underwent acute PVN inhibition by bilateral injection of the GABA-A receptor agonist muscimol (0.1 nmol in 50 nl). Consistent with previous studies, muscimol had no effect in EH rats (n = 6), but reduced mean AP (MAP; P < 0.001) and integrated splanchnic SNA (sSNA; P < 0.001) in DH rats (n = 6). Arterial pulse pressure was unaffected in both groups. Muscimol reduced burst frequency of phrenic nerve activity (P < 0.05) equally in both groups without affecting the burst amplitude-duration integral (i.e. area under the curve). PVN inhibition did not affect the amplitude of the inspiratory peak, expiratory trough or expiratory peak of sSNA in either group, but reduced cardiac rhythmic sSNA in DH rats only (P < 0.001). The latter was largely reversed by inflating an aortic cuff to restore MAP (n = 5), suggesting that the muscimol-induced reduction of cardiac rhythmic sSNA in DH rats was an indirect effect of reducing MAP and thus arterial baroreceptor input. We conclude that MAP is largely maintained in anaesthetized DH rats by a PVN-driven component of sSNA that is neither respiratory nor cardiac rhythmic.

Sympathoexcitation and pressor responses induced by ethanol in the central nucleus of amygdala involves activation of NMDA receptors in rats.

The central nervous system plays an important role in regulating sympathetic outflow and arterial pressure in response to ethanol exposure. However, the underlying neural mechanisms have not been fully understood. In the present study, we tested the hypothesis that injection of ethanol in the central nucleus of the amygdala (CeA) increases sympathetic outflow, which may require the activation of local ionotropic excitatory amino acid receptors. In anesthetized rats, CeA injection of ethanol (0, 0.17, and 1.7 μmol) increased splanchnic sympathetic nerve activity (SSNA), lumbar sympathetic nerve activity (LSNA), and mean arterial pressure (MAP) in a dose-dependent manner. A cocktail containing ethanol (1.7 μmol) and kynurenate (KYN), an ionotropic excitatory amino acid receptor blocker, showed significantly blunted sympathoexcitatory and pressor responses compared with those elicited by CeA-injected ethanol alone (P < 0.01). A cocktail containing ethanol and d-2-amino-5-phosphonovalerate, an N-methyl-d-aspartate (NMDA) receptor antagonist, elicited attenuated sympathoexcitatory and pressor responses that were significantly less than ethanol alone (P < 0.01). In addition, CeA injection of acetate (0.20 μmol, n = 7), an ethanol metabolite, consistently elicited sympathoexcitatory and pressor responses, which were effectively blocked by d-2-amino-5-phosphonovalerate (n = 9, P < 0.05). Inhibition of neuronal activity of the rostral ventrolateral medulla (RVLM) with KYN significantly (P < 0.01) attenuated sympathoexcitatory responses elicited by CeA-injected ethanol. Double labeling of immune fluorescence showed NMDA NR1 receptor expression in CeA neurons projecting to the RVLM. We conclude that ethanol and acetate increase sympathetic outflow and arterial pressure, which may involve the activation of NMDA receptors in CeA neurons projecting to the RVLM.

Dietary salt intake exaggerates sympathetic reflexes and increases blood pressure variability in normotensive rats.

Previous studies have reported that chronic increases in dietary salt intake enhance sympathetic nerve activity and arterial blood pressure (ABP) responses evoked from brain stem nuclei of normotensive, salt-resistant rats. The purpose of the present study was to determine whether this sensitization results in exaggerated sympathetic nerve activity and ABP responses during activation of various cardiovascular reflexes and also increases ABP variability. Male Sprague-Dawley rats were fed 0.1% NaCl chow (low), 0.5% NaCl chow (medium), 4.0% NaCl chow (high) for 14 to 17 days. Then, the animals were prepared for recordings of lumbar, renal, and splanchnic sympathetic nerve activity and ABP. The level of dietary salt intake directly correlated with the magnitude of sympathetic nerve activity and ABP responses to electrical stimulation of sciatic afferents or intracerebroventricular infusion of 0.6 mol/L or 1.0 mol/L NaCl. Similarly, there was a direct correlation between the level of dietary salt intake and the sympathoinhibitory responses produced by acute volume expansion and stimulation of the aortic depressor nerve or cervical vagal afferents. In contrast, dietary salt intake did not affect the sympathetic and ABP responses to chemoreflex activation produced by hypoxia or hypercapnia. Chronic lesion of the anteroventral third ventricle region eliminated the ability of dietary salt intake to modulate these cardiovascular reflexes. Finally, rats chronically instrumented with telemetry units indicate that increased dietary salt intake elevated blood pressure variability but not mean ABP. These findings indicate that dietary salt intake works through the forebrain hypothalamus to modulate various centrally mediated cardiovascular reflexes and increase blood pressure variability.

Respiratory and sympathetic chemoreflex regulation by Kölliker-Fuse neurons in rats

Chemoreceptor activation increases phrenic nerve activity (PNA) and sympathetic nerve activity (SNA). The dorsolateral pontine neurons, including the parabrachial nucleus and the Kölliker-Fuse (KF) region project to several brainstem areas involved in autonomic and respiratory regulation. Here the objective was to further test the hypothesis that the KF region could contribute to central and peripheral sympathetic chemoreflex activation. In urethane-anesthetized sino-aortic denervated or intact and vagotomized male Wistar rats (N = 7-8/group), hypercapnia (end-expiratory CO2 from 5 to 10 %) or KCN increased mean arterial pressure (MAP), splanchnic SNA, and PNA frequency and amplitude. Bilateral injection of muscimol (GABA-A agonist; 2 mM-50 nl) into the KF region increased resting PNA amplitude and reduced resting PNA frequency, without significant changes in resting MAP and SNA. Bilateral blockade of the KF region reduced the rise in MAP, sSNA, and PNA frequency and amplitude produced by hypercapnia or hypoxia. Our data suggest that the KF neurons could integrate and modulate breathing and sympathetic outflow during chemoreceptor activation.

Sustained reduction in blood pressure from electrical activation of the baroreflex is mediated via the central pathway of unmyelinated baroreceptors

AimsThis study aims to identify the contribution of myelinated (A-fiber) and unmyelinated (C-fiber) baroreceptor central pathways to the baroreflex control of sympathetic nerve activity and arterial pressure. Main methodsTwo binary white noise stimulation protocols were used to electrically stimulate the aortic depressor nerve and activate reflex responses from either A-fiber (3V, 20–100Hz) or C-fiber (20V, 0–10Hz) baroreceptor in anesthetized Sprague-Dawley rats (n=10). Transfer function analysis was performed between stimulation and sympathetic nerve activity (central arc), sympathetic nerve activity and arterial pressure (peripheral arc), and stimulation and arterial pressure (Stim-AP arc). Key findingsThe central arc transfer function from nerve stimulation to splanchnic sympathetic nerve activity displayed derivative characteristics for both stimulation protocols. However, the modeled steady-state gain (0.28±0.04 vs. 4.01±0.2%·Hz−1, P<0.001) and coherence at 0.01Hz (0.44±0.05 vs. 0.81±0.03, P<0.05) were significantly lower for A-fiber stimulation compared with C-fiber stimulation. The slope of the dynamic gain was higher for A-fiber stimulation (14.82±1.02 vs. 7.21±0.79dB·decade−1, P<0.001). The steady-state gain of the Stim-AP arc was also significantly lower for A-fiber stimulation compared with C-fiber stimulation (0.23±0.05 vs. 3.05±0.31mmHg·Hz−1, P<0.001). SignificanceThese data indicate that the A-fiber central pathway contributes to high frequency arterial pressure regulation and the C-fiber central pathway provides more sustained changes in sympathetic nerve activity and arterial pressure. A sustained reduction in arterial pressure from electrical stimulation of arterial baroreceptor afferents is likely mediated through the C-fiber central pathway.

Nucleus tractus solitarii reactive oxygen species contribute to acute intermittent hypoxia‐induced long‐term facilitation of phrenic and splanchnic sympathetic nerve activity (686.26)

Cardiorespiratory neural networks undergo alterations in function (plasticity) that are critical to homeostasis but can be deleterious, as in obstructive sleep apnea (OSA). AIH in rats (10X: 45 sec of 10% O2 every 5 min) mimics cardiorespiratory changes as in OSA; prolonged increases in ventilatory/phrenic and sympathetic activity (AIH‐LTF) and augmented hypoxia (Hx) responses post‐AIH. The nTS integrates and modulates primary cardiorespiratory input and undergoes plasticity in response to changes in afferent input, including to Hx. nTS ROS are increased by intermittent Hx and important for plasticity. We hypothesized that nTS ROS contribute to AIH‐LTF and augmented Hx responses post‐AIH. Bilateral microinjection of the antioxidant catalase (CAT, 500 U/ml; 60 nl) in the nTS prior to each Hx bout during AIH attenuated AIH‐LTF for both integrated phrenic amplitude and splanchnic sympathetic nerve activity (%Δ: PhrA, Con=152±58, n=9; CAT=27±19, n=6; SSNA, Con=51±6, n=9; CAT=34±7, n=8). CAT also diminished augmented Hx responses post‐AIH (%Δ vs. Hx1st: PhrA, Con=76±37; CAT=14±22; SSNA, Con=90±11; CAT=50±13). In separate animals, chronically decreasing nTS ROS by injecting a viral vector overexpressing human catalase (AdCAT) also attenuated AIH‐LTF (%Δ: PhrA, Con=77±23, n=8; AdCAT=‐21±15, n=4; SSNA, Con=27±8; AdCAT=1±17). Data suggest nTS ROS contribute to phrenic and sympathetic AIH‐LTF. HL98602Grant Funding Source: Supported by HL98602

Cardiovascular responses to disinhibition of rostral ventrolateral medulla (RVLM) following unilateral RVLM blockade in sedentary or physically active, sinoaortic denervated rats (875.13)

The RVLM is a bilateral region in the brainstem important for sympathetic control of arterial pressure (AP). Although tonically active, both RVLMs receive strong γ‐amino‐butyric acid (GABA)‐mediated inhibition, due in part to tonic arterial baroreceptor input. Previous studies report that unilateral inhibition of the RVLM in sinoaortic denervated (SAD) rabbits leads to dramatic decreases in AP and sympathetic nerve activity (SNA), demonstrating the importance of the baroreflex to resting AP. Following SAD, a strong GABAergic input to RVLM persists and likely suppresses the activity of the RVLM in the absence of baroreceptor input. However, whether this inhibition is altered in sedentary (SED) and active conditions is unknown. The current study was aimed at testing the hypothesis that inhibition of one RVLM in SAD rats would result in enhanced non‐barosensitive GABAergic input in the remaining intact RVLM, and this input would be greater in the SEDs compared to the active rats (i.e., 12 weeks of chronic wheel running on average =WR ). In Inactin‐anesthetized SEDs (n=4) and WRs (n=4), AP and splanchnic SNA were recorded following surgical SAD. Following inhibition of one RVLM (muscimol 2mM, 90nl), bicuculline (5mM, 90nl) was microinjected into the contralateral RVLM. Preliminary data indicate that changes in mean AP appeared to be higher in the SEDs (∆60±12mmHg) compared to the WRs (∆44±15mmHg). Due to small sample size, however, we were unable to support our finding statistically. Splanchnic SNA responses between SEDs (∆52%±16) and WRs (∆49%±23) did not appear to differ. These preliminary data suggest that in the absence of baroreceptor input, MAP and SNA are significantly reduced due to strong non‐barosensitive GABAergic input to the intact RVLM. However, non‐barosensitive GABAergic input may be altered in response to sedentary and active conditions.Grant Funding Source: Supported by R01‐HL096787; R01‐ HL096787‐S1

Chronic vagal nerve stimulation improves baroreflex neural arc function in heart failure rats

We tested whether 6-wk vagal stimulation (VS) treatment improved open-loop baroreflex function in rats after myocardial infarction (MI). The following three groups of Sprague-Dawley rats were examined: normal control (NC, n = 9), MI with no treatment (MI-NT, n = 8), and MI treated with VS (MI-VS, n = 7). Under anesthesia, a stepwise input ranging from 60 to 180 mmHg was imposed on isolated carotid sinus baroreceptor regions, while the responses in splanchnic sympathetic nerve activity (SNA) and arterial pressure (AP) were measured. The response range of percent SNA was greater in the MI-VS than in the MI-NT group (63.8 ± 4.9% vs. 33.1 ± 3.8%, P < 0.01). The slope of the AP response to percent SNA was not different between the MI-VS and MI-NT groups (0.611 ± 0.076 vs. 0.781 ± 0.057 mmHg/%). The difference in the response range of AP between the MI-VS and MI-NT groups did not reach statistical significance (40.7 ± 6.2 vs. 26.4 ± 3.5 mmHg). In conclusion, the 6-wk VS treatment significantly improved the baroreflex control of SNA, but the effect was limited for the baroreflex total-loop function due to the lack of significant improvement in the AP response to percent SNA.

Chapter 10 - Cardiorespiratory Coupling: Common Rhythms in Cardiac, Sympathetic, and Respiratory Activities

Cardiorespiratory coupling is an encompassing term describing more than the well-recognized influences of respiration on heart rate and blood pressure. Our data indicate that cardiorespiratory coupling reflects a reciprocal interaction between autonomic and respiratory control systems, and the cardiovascular system modulates the ventilatory pattern as well. For example, cardioventilatory coupling refers to the influence of heart beats and arterial pulse pressure on respiration and is the tendency for the next inspiration to start at a preferred latency after the last heart beat in expiration. Multiple complementary, well-described mechanisms mediate respiration's influence on cardiovascular function, whereas mechanisms mediating the cardiovascular system's influence on respiration may only be through the baroreceptors but are just being identified. Our review will describe a differential effect of conditioning rats with either chronic intermittent or sustained hypoxia on sympathetic nerve activity but also on ventilatory pattern variability. Both intermittent and sustained hypoxia increase sympathetic nerve activity after 2 weeks but affect sympatho-respiratory coupling differentially. Intermittent hypoxia enhances sympatho-respiratory coupling, which is associated with low variability in the ventilatory pattern. In contrast, after constant hypobaric hypoxia, 1-to-1 coupling between bursts of sympathetic and phrenic nerve activity is replaced by 2-to-3 coupling. This change in coupling pattern is associated with increased variability of the ventilatory pattern. After baro-denervating hypobaric hypoxic-conditioned rats, splanchnic sympathetic nerve activity becomes tonic (distinct bursts are absent) with decreases during phrenic nerve bursts and ventilatory pattern becomes regular. Thus, conditioning rats to either intermittent or sustained hypoxia accentuates the reciprocal nature of cardiorespiratory coupling. Finally, identifying a compelling physiologic purpose for cardiorespiratory coupling is the biggest barrier for recognizing its significance. Cardiorespiratory coupling has only a small effect on the efficiency of gas exchange; rather, we propose that cardiorespiratory control system may act as weakly coupled oscillator to maintain rhythms within a bounded variability.

Ang II–Salt Hypertension Depends on Neuronal Activity in the Hypothalamic Paraventricular Nucleus but Not on Local Actions of Tumor Necrosis Factor-α

Development of angiotensin II (Ang II)-dependent hypertension involves microglial activation and proinflammatory cytokine actions in the hypothalamic paraventricular nucleus (PVN). Cytokines activate receptor signaling pathways that can both acutely grade neuronal discharge and trigger long-term adaptive changes that modulate neuronal excitability through gene transcription. Here, we investigated contributions of PVN cytokines to maintenance of hypertension induced by subcutaneous infusion of Ang II (150 ng/kg per min) for 14 days in rats consuming a 2% NaCl diet. Results indicate that bilateral PVN inhibition with the GABA-A receptor agonist muscimol (100 pmol/50 nL) caused significantly greater reductions of renal and splanchnic sympathetic nerve activity (SNA) and mean arterial pressure in hypertensive than in normotensive rats (P<0.01). Thus, ongoing PVN neuronal activity seems required for support of hypertension. Next, the role of the prototypical cytokine tumor necrosis factor-α was investigated. Whereas PVN injection of tumor necrosis factor-α (0.3 pmol/50 nL) acutely increased lumbar and splanchnic SNA and mean arterial pressure, interfering with endogenous tumor necrosis factor-α by injection of etanercept (10 μg/50 nL) was without effect in hypertensive and normotensive rats. Next, we determined that although microglial activation in PVN was increased in hypertensive rats, bilateral injections of minocycline (0.5 μg/50 nL), an inhibitor of microglial activation, failed to reduce lumbar or splanchnic SNA or mean arterial pressure in hypertensive or in normotensive rats. Collectively, these findings indicate that established Ang II-salt hypertension is supported by PVN neuronal activity, but short term maintenance of SNA and arterial blood pressure does not depend on ongoing local actions of tumor necrosis factor-α.

Respiratory, metabolic and cardiac functions are altered by disinhibition of subregions of the medial prefrontal cortex

The prefrontal cortex (PFC) is referred to as the visceral motor cortex; however, little is known about whether this region influences respiratory or metabolic outflows. The aim of this study was to describe simultaneous changes in respiratory, metabolic and cardiovascular functions evoked by disinhibition of the medial PFC (mPFC) and adjacent lateral septal nucleus (LSN). In urethane-anaesthetized rats, bicuculline methiodide was microinjected (2 mm; GABA-A receptor antagonist) into 90 sites in the mPFC at 0.72-4.00 mm from bregma. Phrenic nerve amplitude and frequency, arterial pressure, heart rate, splanchnic and lumbar sympathetic nerve activities (SNA), expired CO2, and core and brown adipose tissue temperatures were measured. Novel findings included disturbances to respiratory rhythm evoked from all subregions of the mPFC. Injections into the cingulate cortex evoked reductions in central respiratory function exclusively, whereas in ventral sites, particularly the infralimbic region, increases in respiratory drive and frequency, and metabolic and cardiac outflows were evoked. Disinhibition of sites in surrounding regions revealed that the LSN could evoke cardiovascular changes accompanied by distinct oscillations in SNA, as well as increases in respiratory amplitude. We show that activation of neurons within the mPFC and LSN influence respiratory, metabolic and cardiac outflows in a site-dependent manner. This study has implications with respect to the altered PFC neuronal activity seen in stress-related and mental health disorders, and suggests how basic physiological systems may be affected.

Abstract 445: Increased Dietary Salt Intake Enhances Sympathetic and Cardiovascular Responses to Various Stressors

Previous studies indicate that increased dietary salt intake enhances sympathetic nerve activity (SNA) and arterial blood pressure (ABP) responses evoked from sympathetic neurons of the rostral ventrolateral medulla. The present study sought to extend these findings and determine whether dietary salt intake enhances SNA and ABP responses various sympathetic reflexes that depend on RVLM neurotransmission. Male Sprague-Dawley rats were fed 0.1% (n=6-8) or 4.0% (n=6-8) NaCl diets for 14-21 days. Then, animals were anesthetized with Inactin. Electrical stimulation (1-20 Hz, 1 ms pulse, 500 uA) of sciatic afferents produced frequency-dependent changes in SNA and ABP in both groups. However, rats ingesting 4% versus 0.1% NaCl displayed significantly larger increases in lumbar SNA (5Hz: 213±25 vs 146±25%, P&lt;0.05), renal SNA (5Hz: 187±24 vs 120±11%), splanchnic SNA (5Hz: 203±21 vs 136±9%), and mean ABP (5Hz: 28±2 vs 12±2 mmHg). Rats ingesting 4% vs 0.1% NaCl also displayed greater increases in lumbar SNA (24±6 vs 13±2%, P&lt;0.05) and mean ABP (12.1±0.9 vs 8.2±1.3mmHg, P&lt;0.05) during increases in cerebrospinal fluid sodium concentration produced by intracerebroventricular infusion of 1M NaCl (5ul/10min). Lastly, hypercapnia (7% CO2, 33% O2, 63% N2, 60s) produced greater increases in lumbar SNA in rats ingesting 4% versus 0.1% NaCl (24±2% versus 9±3%, respectively; P&lt;0.01). These findings suggest increased dietary salt intake enhances several sympathetic and cardiovascular reflexes.

Abstract 446: Rostral Ventrolateral Lateral Medulla Mediates the Sympathetic and Cardiovascular Response to Increased Cerebrospinal Fluid Sodium Concentration

Compelling evidence indicates increased cererbrospinal fluid (CSF) sodium elevates sympathetic nerve activity (SNA) and arterial blood pressure (ABP) in salt-sensitive hypertension. RVLM neurons project to the spinal cord and regulate SNA and ABP under a number of physiological challenges and pathophysiological states. Therefore, we hypothesized that RVLM neurons mediate the sympathoexcitatory response to increased CSF sodium. Inactin-anesthetized, male Sprague-Dawley rats (n=5/group) were prepared for sympathetic nerve recordings and a lateral ventricle brain cannula. Infusion of 1M NaCl (5 μL/10 min) to increase CSF sodium concentrations by 5mM produced a significant (P’s&lt;0.05, n=6) increase in lumbar SNA (Δ: 115±3%), adrenal SNA (Δ: 122±3%), and mean arterial blood pressure (Δ: 8±1 mmHg). The infusion did not affect splanchnic SNA (Δ: 102±4%) but decreased renal SNA (Δ: 91±2%). Inhibition of the RVLM with bilateral injection of the GABA agonist muscimol (2.5mM per 50 nL per side) significantly (P’s&lt;0.05; n=5) attenuated the increased lumbar SNA (Δ:101±2%), adrenal SNA (Δ:105±2%), and mean arterial blood pressure (Δ: 1±1mmHg, P’s&lt;0.05; n=6). Blockade of ionotropic glutamate receptors in the RVLM with bilateral injection of kynurenic acid (30mM per 50 nL per side) also significantly (P’s&lt;0.05; n=5) attenuated the increase in lumbar SNA (Δ: 101±3%), adrenal SNA (Δ: 110±3%) and mean ABP (Δ: 1±2 mmHg) to lateral ventricle infusion of 1M NaCl (5uL/10 min). In a final set of experiments, in vivo single-unit recordings demonstrate that ventricular infusion of 1M NaCl (5uL per 10 min) increased discharge in 60% (6/10) of spinally-projecting, barosensitive RVLM neurons (2.1±0.4 to 5.8±0.2 Hz, P&lt;0.05). Infusion of artificial CSF did not affect any variable. These findings suggest increased CSF sodium activates a glutamatergic pathway to RVLM neurons to elevate SNA and ABP.

Commissural nucleus of the solitary tract regulates the antihypertensive effects elicited by moxonidine

The rostral ventrolateral medulla (RVLM) contains the presympathetic neurons involved in cardiovascular regulation that has been implicated as one of the most important central sites for the antihypertensive action of moxonidine (an α2-adrenergic and imidazoline agonist). Here, we sought to evaluate the cardiovascular effects produced by moxonidine injected into another important brainstem site, the commissural nucleus of the solitary tract (commNTS). Mean arterial pressure (MAP), heart rate (HR), splanchnic sympathetic nerve activity (sSNA) and activity of putative sympathoexcitatory vasomotor neurons of the RVLM were recorded in conscious or urethane-anesthetized, and artificial ventilated male Wistar rats. In conscious or anesthetized rats, moxonidine (2.5 and 5nmol/50nl) injected into the commNTS reduced MAP, HR and sSNA. The injection of moxonidine into the commNTS also elicited a reduction of 28% in the activity of sympathoexcitatory vasomotor neurons of the RVLM. To further assess the notion that moxonidine could act in another brainstem area to elicit the antihypertensive effects, a group with electrolytic lesions of the commNTS or sham and with stainless steel guide-cannulas implanted into the 4th V were used. In the sham group, moxonidine (20nmol/1μl) injected into 4th V decreased MAP and HR. The hypotension but not the bradycardia produced by moxonidine into the 4th V was reduced in acute (1day) commNTS-lesioned rats. These data suggest that moxonidine can certainly act in other brainstem regions, such as commNTS to produce its beneficial therapeutic effects, such as hypotension and reduction in sympathetic nerve activity.

Rostroventrolateral medulla neurons with commissural projections provide input to sympathetic premotor neurons: anatomical and functional evidence

The activity of neurons in the rostral ventrolateral medulla (RVLM) is critical for the generation of vasomotor sympathetic tone. Multiple pre-sympathetic pathways converge on spinally projecting RVLM neurons, but the origin and circumstances in which such inputs are active are poorly understood. We have previously shown that input from the contralateral brainstem contributes to the baseline activity of this population: in the current study we investigate the distribution, phenotype and functional properties of RVLM neurons with commissural projections in the rat. We firstly used retrograde transport of fluorescent microspheres to identify neurons that project to the contralateral RVLM. Labelled neurons were prominent in a longitudinal column that extended over 1 mm caudal from the facial nucleus and contained hybridisation products indicating enkephalin (27%), GABA (15%) and adrenaline (3%) synthesis and included 6% of bulbospinal neurons identified by transport of cholera toxin B. Anterograde transport of fluorescent dextran-conjugate from the contralateral RVLM revealed extensive inputs throughout the RVLM that frequently terminated in close apposition with catecholaminergic and bulbospinal neurons. In urethane-anaesthetised rats we verified that 28/37 neurons antidromically activated by electrical stimulation of the contralateral pressor region were spontaneously active, of which 13 had activity locked to central respiratory drive and 15 displayed ongoing tonic discharge. In six tonically active neurons sympathoexcitatory roles were indicated by spike-triggered averages of splanchnic sympathetic nerve activity. We conclude that neurons in the RVLM project to the contralateral brainstem, form synapses with sympathetic premotor neurons, and have functional properties consistent with sympthoexcitatory function.